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I am having problems with the su command. I know my password and I am typing it correctly but su indicates authentication failure.

So I checked on the internet and then went into recovery mode and changed my username's password to what I was entering before.

Even now entering the same password on su gives me authentication failure.

Please suggest what am I doing wrong??

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up vote 25 down vote accepted

su asks for the root password. Since Ubuntu doesn't set a root password by default, you can't use it to become root.

Instead, to become root, use sudo -i with your personal password.

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I admit I tend to use sudo bash but your incantation makes me want to use su sudo or maybe su su sudo. – Jerry Apr 10 '11 at 9:42
Interesting, thank you. – Jerry Apr 10 '11 at 17:49
sudo su is not the recommended way - use sudo -i instead. – guntbert Mar 2 '14 at 21:30

su asks for the password of the account you're trying to login. It's usage (simplified):

su username

When omitting username, the username default to root. Since the root password is disabled by default on Ubuntu, no password will be valid. The preferred way to run root commands is not through a su shell, but with sudo as in:

sudo mount /dev/sdb1 /mnt
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The root account in Ubuntu is disabled by default. This is to say that it has no password (which is different from it having a blank password) and no attempt to authenticate with root's password will succeed. Therefore, su or su - will not work.

Instead, use sudo to run a command as root:

sudo command...

If you want a root shell like you get with su, run:

sudo -s

If you want a root shell like you get with su -, run:

sudo -i
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su asks for the root password.

You can set a root password while you're root (by giving sudo su, providing that you are on the sudoers file), by giving the command passwd and setting a new password.

This is not recommend it for various reasons, though.

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I came across an OS, Minibian, where the setuid bit was missing from /bin/su, causing this error, even if the password of the root account was enabled.

On Linux, the only way to become root is to execute a setuid-root file. When you run sudo or su, their setuid bit is set, so the process' effective user becomes root, then they do the authentiaction already as root! If it fails they exit.

Anyway, here are the symptoms and the fix:

$ su
su: authentication failure
$ sudo su
# ls -l /bin/su
-rwxr-xr-x 1 root root 31092 Jun  5  2012 /bin/su
# chmod u+s /bin/su
# ls -l /bin/su
-rwsr-xr-x 1 root root 31092 Jun  5  2012 /bin/su

Note the difference: rwx before, rws after.

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I don't really see how a bug on a completely different Linux distribution is related to Ubuntu...? – Byte Commander Feb 19 at 8:20

In my case, it was because the entry for that user was missing in /etc/shadow.

I had copied to another test server all entries in /etc/passwd with ID higher than 1000 along with /etc/group but forgot /etc/shadow. So everytime I did su with any of those users I would get that error. After adding the missing entry in /etc/shadow the error would stop appearing.

For example /etc/shadow:

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In Ubuntu root account is disabled by default. In order to perform root action you can do one of the following

  1. Use sudo with your user password, and you can do everything that root user do.
  2. Use sudo su with your user password to obtain root access
  3. Use sudo passwd with that command are changing the root password, then you can easily access root user using su like other distros (and you can continue to use also sudo).
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